Rotary actuator

ABSTRACT

A rotary actuator ( 100 ) having a reference structure ( 110 ), an output member ( 113 ) arranged for rotary movement relative to the reference structure, a first linear motor ( 116 ) arranged to selectively apply an output force urging a first motor member ( 119 ) and a second motor member ( 122 ) apart along a generally linear direction, in which the first linear motor is configured and arranged to cause a torque between the output member and the reference structure in a first direction, and second linear motor ( 137 ) arranged to selectively apply an output force urging a second linear motor first member ( 134 ) and a second motor member ( 137 ) apart along a generally linear direction, in which the second linear motor is configured and arranged to cause a torque between the output member and the reference structure in a direction opposite to the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication No. 61/597,141 which was filed on Feb. 9, 2012, which ishereby incorporated by reference.

TECHNICAL FIELD

The present invention relates generally to the field of rotaryactuators, and more specifically to high performance small size rotaryactuators.

BACKGROUND ART

Several types of rotary actuators are generally known. For example,vane-based rotary hydraulic actuators have been produced as well aspurely electric motor based rotary actuators.

BRIEF SUMMARY OF THE INVENTION

With parenthetical reference to the corresponding parts, portions orsurfaces of the disclosed embodiment, merely for the purposes ofillustration and not by way of limitation, provided is a rotary actuator(100) having a reference structure (110), an output member (113)arranged for rotary movement relative to the reference structure aboutan axis, a first linear motor (116) having a first member (119) and asecond member (122), the first linear motor configured and arranged toselectively apply an output force urging the first member and the secondmember apart along a generally linear direction, the first linear motorfirst member coupled to the reference structure, the first linear motorsecond member coupled to the output member and configured and arrangedto cause a torque between the output member and the reference structurein a first direction about the axis when the first linear motor appliesthe output force, a second linear motor (131) having a first member(134) and a second member (137), the second linear motor configured andarranged to selectively apply an output force urging the second linearmotor first member and the second linear motor second member apart alonga generally linear direction, the second linear motor first membercoupled to the reference structure, and the second linear motor secondmember coupled to the output member and configured and arranged to causea torque between the output member and the reference structure in adirection about the axis opposite the first direction when the secondlinear motor output force is applied.

The first linear motor (216) may comprise a single acting hydraulicmotor. The first linear motor first member may have a prismatic chamberand/or the first linear motor second member may have a piston (222). Theprismatic chamber may be a cylinder (219). The first linear motor mayhave a piston link (248) arranged between the piston and the outputmember. The first linear motor first member may be rigidly mounted tothe reference structure. The piston link and/or the piston may beconnected through a ball joint. The piston link and the output membermay be connected through a pivot joint (228) or pin joint. The firstlinear motor and the second linear motor may each have a direction ofaction which may be generally parallel. The output member may have ashaft. The output member may have a first pivot bearing (228) coupled tothe first motor second member and/or a second pivot bearing (240)coupled to the second motor second member. The first pivot bearing andthe second pivot bearing may be separated by an offset in a dimensionparallel to the axis.

The first pivot bearing, the second pivot bearing, and the axis may becollinear. The first linear motor first member may have a cylinder andthe first linear motor second member may have a piston, the pistonhaving a first surface and a second surface. The first surface may forma first chamber (245) with the cylinder and the second surface form asecond chamber (255) with the cylinder. The cylinder may have agenerally cylindrical surface. The cylindrical surface may have a holebetween the piston first surface and the piston second surface. Therotary actuator may further have a drive link coupled to the piston. Thedrive link may traverse the hole.

The second linear motor (231) may have a single acting hydraulic motor.The first linear motor and the second linear motor may have anequivalent hydraulic fluid volume displaced for a given linear motorlinear distance of actuation. The first linear motor and the secondlinear motor may be hydraulically balanced. The output member may becoupled to an aircraft control surface. The rotary actuator may furtherhave a position sensor configured and arranged to measure an anglebetween the output member and the reference structure. The rotaryactuator may further have a servo controller.

In another aspect, provided is an actuator (300) for rotating a shaft(313) about an axis (319), which has: a housing (303), a first singleacting cylinder (322) disposed in the housing and having a first piston(328) and a first connecting link (349) therein, a crank (334) attachedto the shaft, a second single acting cylinder (325) disposed in thehousing and having a second piston (331) and a second connecting link(349) therein, in which the first and second connecting links may beattached to different locations on the crank, and in which the actuatoris configured and arranged such that actuation of the first actingcylinder causes the crank to rotate in a first direction and actuationof the second single acting cylinder causes the crank to rotate in asecond direction opposite the first direction.

The first and second cylinders may be oriented substantially parallel.The first and second cylinders may be both configured and arranged toeach have a pre-load to provide a force in the same general direction toremove a backlash. The shaft may rotate on a set of bearings disposed inthe housing. The shaft may be connected to an aircraft control surface.The actuator may be configured and arranged to move the crank from afirst position to a second position by applying an additional pressureto one of a first pressure chamber and a second pressure chamber. Theactuator may be configured and arranged to be able to maintain aposition of the crank by providing a substantially equal pressure insidethe first and second pressure chambers.

The actuator may be configured and arranged to maintain a position ofthe crank by not allowing hydraulic fluid to flow in or out of the firstor second pressure chambers. The first and second single actingcylinders may have a cross section which may be not circular. The firstconnecting link may be connected to the first piston through a balljoint (352). The first connecting link and the output member may beconnected through a pivot joint or pin joint. The first and secondsingle acting cylinders may be separated by an offset in a dimensionparallel to the axis. The first and second single acting cylinders mayshare a common bore. The actuator may be configured and arranged suchthat the first single acting cylinder expels a substantially equalvolume of hydraulic fluid to a volume of hydraulic fluid drawn in by thesecond single acting cylinder for a movement of the shaft. The actuatormay further have a position sensor arranged and configured to measure anangle between the shaft and the housing. The actuator may further have aservo controller.

In another aspect, provided is a method of operating an actuator havinga reference structure (110), an output member (113) arranged for rotarymovement relative to the reference structure about an axis, a firstlinear motor (116) having a first member (119) and a second member(122), the first linear motor configured and arranged to selectivelyapply an output force urging the first member and the second memberapart along a generally linear direction, the first linear motor firstmember coupled to the reference structure, the first linear motor secondmember coupled to the output member and configured and arranged to causea torque between the output member and the reference structure in afirst direction about the axis when the first linear motor applies theoutput force, a second linear motor (131) having a first member (134)and a second member (137), the second linear motor configured andarranged to selectively apply an output force urging the second linearmotor first member and the second linear motor second member apart alonga generally linear direction, the second linear motor first membercoupled to the reference structure, the second linear motor secondmember coupled to the output member and configured and arranged to causea torque between the output member and the reference structure in adirection about the axis opposite the first direction when the secondlinear motor output force may be applied, having the steps of: causingthe first linear motor to apply a first non-zero force and the secondlinear motor to apply a second non-zero force, whereby a backlash may bereduced.

The method may further have the steps of: receiving a commanded outputmember characteristic, and adjusting the first non-zero force relativeto the second non zero force when the output member has an actualcharacteristic which does not match the commanded output membercharacteristic. The output member characteristic may be an anglerelative to the reference structure. The method may further have thestep of increasing the second non-zero force relative to the first nonzero force when the output member has an angle less than the commandedoutput member angle position, whereby a torque may be applied betweenthe output member and the reference structure.

In another aspect, provided is a hydraulic actuator (400) having acylinder (419) having a generally cylindrical shaped inner surface witha longitudinal axis and a first end and a second end, the inner surfacehaving a hole (470) arranged between the first end and the second end, apiston (422) configured and arranged for sliding movement within thecylinder, the piston having a first surface and a second surface, thefirst surface and the second surface facing generally oppositedirections along the longitudinal axis, the first surface forming afirst chamber (494) with the cylinder and the second surface forming asecond chamber (495) with the cylinder, a first hydraulic port (492) influid communication with the first chamber, a second hydraulic port(493) in fluid communication with the second chamber, a drive link (448)having a first end and a second end and arranged to pass through thehole, the drive link first end coupled to a position on the pistonbetween the first surface and the second surface, in which the actuatormay be configured and arranged to cause a movement of the drive linkrelative to the cylinder when the piston moves relative to the cylinder.

The drive link does not pass through the first chamber or the secondchamber. The hydraulic actuator may further have a reference structureand a pivot joint between the drive link and the reference structureconfigured and arranged to allow a rotary movement between the drivelink and the reference structure about an axis. The cylinder may bemounted to the reference structure. The hydraulic actuator may furtherhave a drive shaft (413) configured and arranged for rotary movementrelative to the cylinder. The drive shaft may be coupled to the drivelink. The cylinder may have a non-circular cross section. The drive linkmay be coupled to the piston through a pivot or pin joint. The drivelink may be coupled to the piston through a universal joint. The drivelink may be coupled to the piston through a ball joint. The firstchamber and the second chamber may be hydraulically balanced.

The actuator may be configured and arranged such that the first chamberexpels a substantially equal volume of hydraulic fluid to a volume ofhydraulic fluid drawn in by second chamber for a movement of the pistonrelative to the cylinder. The hydraulic actuator may further have aposition sensor configured and arranged to measure an angle between thedrive link and the cylinder. The hydraulic actuator may further have aservo controller.

In another aspect, provided is an actuator power system having a bentaxis hydraulic pump (740) having a first hydraulic port (733), a secondhydraulic port (735), and an input drive shaft, a gear assembly (750)having a gear shaft mechanically coupled to the bent axis pump inputdrive shaft for providing a mechanical advantage to cause the bent axispump to rotate at a lower speed than the gear shaft, in which theactuator power system may be configured and arranged to cause a fluidflow between the first hydraulic port and the second hydraulic port whenthe gear shaft may be rotated. The actuator power system may furtherhave an electric motor (760) coupled to the gear shaft. The actuatorpower system may further have a hydraulically balanced rotary actuatorconfigured and arranged to be powered from the bent axis hydraulic pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a first embodiment of the rotary actuator.

FIG. 2 is a side view of a second embodiment of the rotary actuator.

FIG. 3 is a perspective of a third embodiment of the rotary actuator.

FIG. 4 is a front view of the rotary actuator shown in FIG. 3.

FIG. 5 is a side view of the rotary actuator shown in FIG. 3.

FIG. 6 is a perspective view of the embodiment shown in FIG. 3 with itscase removed.

FIG. 7 is a front view of the embodiment shown in FIG. 6.

FIG. 8 is a side view of the embodiment shown in FIG. 6.

FIG. 9 is an elevational view of a piston and connecting rod assembly ofthe rotary actuator shown in FIG. 6.

FIG. 10A is a cross-sectional view taken along lines 10A-10A in FIG. 9.

FIG. 10B is an exploded perspective view of a portion of the assemblyshown in FIG. 10A.

FIG. 11 is a side section view of a fourth embodiment of the rotaryactuator taken along lines 11-11 in FIG. 12.

FIG. 12 is a front view of the fourth embodiment of the rotary actuator.

FIG. 13 is a perspective view of the piston assembly of the rotaryactuator shown in FIG. 12.

FIG. 14 is a side section view of the piston assembly shown in FIG. 13.

FIG. 15 is an enlarged view of the circular dashed section shown in FIG.14.

FIG. 16 is a front view of a fifth embodiment of the rotary actuator.

FIG. 17 is a side section view of the rotary actuator shown in FIG. 16.

FIG. 18 is an isometric view of an alternative piston assembly.

FIG. 19 is a top view of the alternative piston assembly shown in FIG.18.

FIG. 20 is a side view of the alternative piston assembly shown in FIG.18.

FIG. 21 is a side section view of the alternative piston assembly shownin FIG. 18.

FIG. 22 is a side view of another embodiment of the rotary actuator withits case removed.

FIG. 23 is a system diagram of a rotary actuator system.

FIG. 24 is a section view of a first version pump shown in FIG. 23.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., cross-hatching, arrangement of parts, proportion,degree, etc.) together with the specification, and are to be considereda portion of the entire written description of this invention. As usedin the following description, the terms “horizontal”, “vertical”,“left”, “right”, “up” and “down”, as well as adjectival and adverbialderivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”,etc.), simply refer to the orientation of the illustrated structure asthe particular drawing figure faces the reader. Similarly, the terms“inwardly” and “outwardly” generally refer to the orientation of asurface relative to its axis of elongation, or axis of rotation, asappropriate.

The disclosed embodiments provide high performance rotary actuators androtary actuator systems, which are driven by linear motors. Referringnow to the drawings, and more particularly to FIG. 1, a side view of afirst embodiment of the rotary actuator is disclosed. Rotary actuator100 includes reference structure 110, first linear motor 116, secondlinear motor 131, and output member 113. As shown in FIG. 1, referencestructure 110 is generally a rigid frame or housing. Output member 113is also a generally rigid structure. Output member 113 is coupled toreference structure 110 through pivot joint 114. Pivot joint 114 allowsrotary movement between reference structure 110 and output element 113about axis 115. (Axis 115, as shown in FIG. 1, has a directionperpendicular to the page.) Pivot joint 114 may also contain a sensorwhich measures the angle and/or torque between reference structure 110and output member 113.

Linear motor 116 has two main parts which include first member 119 andsecond member 122. First member 119 and second member 122 are coupledfor linear movement relative to one another. When linear motor 116 isactivated, a force is applied urging first member 119 and second member122 apart along direction 160.

Linear motor 131 is similar to linear motor 116. Linear motor 131 hastwo main parts, first member 134 and second member 137. First member 134and second member 137 are coupled for linear movement relative to oneanother. When linear motor 131 is activated, a force is applied urgingfirst member 134 and second member 137 apart along direction 163.

First member 119 of linear motor 116 is coupled to reference structure110 at coupling 125. Second member 122 is coupled to output member 113at coupling 128. Couplings 125 and 128 may include a pivot joint,universal joint, or ball joint. Couplings 125 and 128 may also be arigid mounting if a portion of first member 119 and second member 122are able to rotate relative to each other. The actuation of linear motor116 urges second member 122 to be driven rightwards relative to firstmember 119 along line 160. This actuation effectively urges coupling 125and coupling 128 apart. Stated another way, linear motor 119 causes aforce to be applied between reference structure 110 at point 125 andoutput member 113 at point 128 in a direction that urges the two apart.Because coupling 128 is above line 165 formed between coupling 125 andpivot joint 114, the force applied by linear motor 116 causes torque 169between reference structure 110 and output member 113.

Linear motor 131 is similarly connected between reference structure 110and output element 113, however, linear motor 131 is arranged toselectively cause a torque 166 to be applied between reference structure110 and output element 113 which has a direction opposite torque 169.More specifically, first member 134 of linear motor 131 is coupled toreference structure 110 at coupling 143. Second member 137 is coupled tooutput member 113 at coupling 140. Couplings 143 and 140 may include apivot joint, universal joint, ball joint, or may also be a rigidmounting if a portion of first member 134 and second member 137 are ableto rotate relative to each other. The actuation of linear motor 131urges second member 137 to be driven rightwards relative to first member134. This actuation effectively urges coupling 143 and coupling 140apart. Stated another way, linear motor 131 causes a force to be appliedbetween reference structure 110 at point 143 and output member 113 atpoint 140 in a direction that urges the two apart. Because pivot joint140 is below line 167 formed between coupling 143 and pivot joint 114,the force applied by linear motor 131 causes torque 166 betweenreference structure 110 and output member 113.

Because pivot joint 140 is below the line 167, compared to pivot joint128 which is above the line 165, the torques 166, 169 applied by eachrespective linear motor are in opposite directions. Linear motors 116and 131 only need to be able to produce a force in a single directionbetween reference structure 110 and output element 113. There is no needfor linear motors 116 and 131 to be able to provide a force in bothdirections, i.e. both “pushing” and “pulling”. It is only necessary thatlinear motors 116 and 131 are arranged to cause opposite directiontorques to be applied between reference structure 110 and output member113. While only a single acting linear motor is necessary for rotaryactuator 110, dual acting linear motors can be used in rotary actuator110 without departing from the spirit of the invention. Also, linearmotors 116 and 131 can be replaced with single acting linear motorswhich both only provide a “pulling” force instead of a “pushing” force,since the motors would still be able to produce opposite torques.

Linear motors 116 and 131 may be electrical motors, single actinghydraulic actuators, pneumatic actuators, linear drive screw, or anyother similar motor type. Rotary actuator 110 may also include a servocontroller.

Rotary actuator 100 can be operated in multiple different operationalmodes. A first method of operation is a low backlash mode. In the lowbacklash mode, a minimum threshold force is always applied by eachlinear motor 116, 131 while rotary actuator 100 is in use. Under thismode, because the mechanical linkages of the system are always undercompression, any tolerance or “play” in joints 114, 125, 128, 140, and143 will be “forced to one side” of their region of “play” andeffectively prevented from causing a backlash. For example, if rotaryactuator 100 is first commanded to apply a clockwise torque to outputmember 113 and then subsequently apply a counterclockwise torque,because the mechanical linkages in rotary actuator 100 are always incompression, significant backlash will not occur. More specifically, inorder to apply clockwise torque 169 to output member 113, linear motor116 may be commanded to apply 20 N of force while linear motor 131 iscommanded to apply a minimum threshold force of 10 N. A force of 10 N isthus being counteracted between the motors, and this 10 N ofcounteraction force is passed through the mechanical linkage between themotors. Because each element of rotary actuator 100 is therefore undercompression caused by the counteraction force, none of the joints ofrotary actuator 100 will be free to “jiggle.” Next, when rotary actuator100 is commanded to apply a counterclockwise torque 166, linear motor116 is commanded to apply the minimum threshold force of 10 N whilelinear motor 131 is commanded to apply a force of 20N. This causes thenet torque on output member 113 to shift from clockwise tocounterclockwise while all of the joints of rotary actuator 100 remainin compression. A typical prior art rotary actuator which has a singledual acting motor is not capable of maintaining all joints of itsmechanical linkage in compression when switching between applyingclockwise and counterclockwise torques, and therefore suffers frombacklash when individual mechanical linkage joints switch from being incompression and tension.

Rotary actuator 100 can also be operated in a low friction mode, inwhich only one motor is active at a time. By preventing continuoustension in the mechanical linkage which is present in the low backlashmode of operation, the friction experienced by the individual linkagejoints is reduced. The low friction mode of operation helps reduce wearrates of the joints and linear motors. Also, because each motor is notcontinuously on, efficiency increases may also be realized with the lowfriction mode compared to the low backlash mode of operation.

It is also possible to selectively adjust the mode of operation ofrotary actuator 100 depending upon a particular need of the system at agiven time.

FIG. 2 is a side partial section view of a second embodiment rotaryactuator. Rotary actuator 200 includes major components of referenceframe 210, first single acting hydraulic motor 216, second single actinghydraulic motor 231, and output member 213. Rotary actuator 200 isarranged to drive driven member 250, which is rigidly coupled to outputmember 213.

First hydraulic motor 216 has cylinder 219 and piston 222. Cylinder 219is rigidly mounted to reference frame 210. Piston 222 is configured andarranged within cylinder 219 to allow piston 222 to slide left and rightwithin cylinder 219, while maintaining a seal between the outer surfaceof piston 222 and the inner cylindrical wall of cylinder 219. Piston 222and cylinder 219 form chamber 245, which is in fluid communication withhydraulic port 246. Piston 222 has a pivot joint 247, which is coupledto the left end of connecting link 248. The right end of connecting link248 is coupled to output member 213 through pivot joint 228.

Output member 213 is coupled to reference structure 210 through pivotjoint 214, which allows output member 213 to rotate relative toreference structure 210 about axis 215. Pivot joint 214 has rotaryposition sensor 217, which senses the angle between reference structure210 and output member 213 and outputs this angle information on outputline 218. Driven member 250 is rigidly coupled to output member 213 andpivot joint 214 such that driven member 250 rotates together with outputmember 213 relative to reference structure 210.

Second hydraulic motor 231 has cylinder 234, which is rigidly mounted toreference frame 210, and piston 237. Piston 237 is configured andarranged within cylinder 234 to allow piston 237 to slide left and rightwithin cylinder 234, while maintaining a seal between the outer surfaceof piston 237 and the inner cylindrical wall of cylinder 234. Piston 237and cylinder 234 form chamber 255, which is in fluid communication withhydraulic port 256. Piston 237 has a pivot joint 257, which is coupledto the left end of connecting link 258. The right end of connecting link258 is coupled to output member 213 through pivot joint 240.

Hydraulic fluid is provided by ports 246 and 256 to single acting linearhydraulic motors 216 and 231 respectively. The output force produced bylinear motors 216 and 231 is directly dependent upon the pressure of thefluid in ports 246 and 256 respectively. The pressure in ports 246 and256 can be controlled with standard hydraulic valves.

The operation of rotary actuator 200 is substantially similar to rotaryactuator 100. More concretely, hydraulic motors 216 and 231 both produce“pushing” forces between reference structure 210 and output member 213which respectively cause torques of opposite polarity to be appliedbetween reference structure 210 and output member 213. Similarly, in theoperation of rotary actuator 100, motors 119 and 131 produce oppositetorques between reference structure 110 and output member 113. Also,rotary actuator 200 can be operated in a low backlash mode and lowfriction mode similar to the modes of operation for the rotary actuator100.

The dimensions of linear hydraulic motors 216 and 231 are substantiallythe same. More specifically, the cross sectional area of cylinder 219 issubstantially the same as the cross sectional area of cylinder 234.These dimensions cause a volume of hydraulic fluid to flow in throughport 246 when piston 222 is displaced rightwards to be equivalent to thevolume that flows into port 256 for an equivalent rightwardsdisplacement of piston 237. When rotary actuator 200 is “centered”,meaning piston 222 is displaced by an equivalent amount as piston 237,rotation of output member 213 causes a rightwards displacement of piston222 which is substantially equivalent to the leftwards displacement ofpiston 237. This characteristic of “balanced displacement” of rotaryactuator 200 has significant positive implications for the overallhydraulic system used to drive rotary actuator 200. Because the totalhydraulic volume actively in the hydraulic system, excluding thehydraulic reservoir, will remain generally constant in a system havingonly “balanced displacement” actuators such as rotary actuator 200, theefficiency of the system is significantly improved. Compared to anon-balanced hydraulic system, the work potential of high pressure fluidis not lost each time the total in system hydraulic volume decreases.

Turning to FIGS. 3-10, and initially to FIG. 3, third embodiment rotaryactuator 300 has a housing 303 that is formed with a main body 306 thatsurrounds a shaft 313 (best shown in FIGS. 6 and 8). The shaft 313 maybe mounted on bearings (not shown) at opposite ends and may be providedwith an output member 316 that may be integrally formed or attached tothe shaft 313 such that the output member 316 rotates about the centrallongitudinal shaft axis 319. The housing 303 also includes two cylinders322 and 325 extending from main body 306. Cylinders 322 and 325 definechambers for pistons 328, 331 (shown in FIGS. 6 and 8). Pistons 328 and331 have a circular cross-section suitable for use in the cylinders 322and 325. Cylinders 322 and 325 in this embodiment are single actinghydraulic cylinders as described in greater detail below. It will beunderstood by those of ordinary skill in the art based on thisdisclosure that the term cylinder is used to describe the barrel of alinear motor and is not intended to be limited to any specific shape andother shapes of chambers for receiving different shaped pistons wouldalso be suitable. For example, cylinder may refer to a barrel with anon-circular cross section, or a barrel with generally prismatic shape.The cylinders 322 and 325 have longitudinal axes 323, 324 respectivelythat are disposed on opposite sides of the shaft axis 319 as best shownin FIG. 4.

In FIG. 6, housing 303 has been removed for clarity to show thearrangement of pistons 328 and 331. Pistons 328 and 331 are connected tocrankpins 334 and 337 disposed on opposite sides of the centrallongitudinal shaft axis 319. Accordingly, downward movement of piston328 causes the shaft 313 to rotate counterclockwise about axis 319relative to the orientation of FIG. 6 and downward movement of piston331 causes the shaft 313 to rotate clockwise about axis 319 relative tothe orientation of FIG. 6. As best shown in FIG. 5, the cylinders 322and 325 may be mounted at different positions along the length of theshaft 313. Additionally, as best shown in FIG. 4, pistons 322 and 325may be staggered or offset 327 from each other.

Piston 328 has a substantially flat surface 340 at first end 343 thatforms an end wall of the pressure chamber when piston 328 is installedinside cylinder 322. Cylinder 322 is single acting as the portion of thechamber adjacent to surface 340 is the only part exposed to workingfluid. Accordingly, piston 328 only has to be sealed with respect to onepressure chamber, and piston 328 and connecting rod 349 are not sealedat second end 346. Connecting rod 349 is attached to piston 328 by aball and pin structure 352 that is described in greater detail below.Legs 362, 365 of the connecting rod 349 are connected to crankpin 334 onshaft 313 as described in greater detail below. Piston 331 has a flatsurface 332 and is installed in cylinder 331 and connected to crankpin337 by connecting rod 349 in the same manner as piston 328.

Turning to FIG. 9, piston 328 and connecting rod 349 are shown ingreater detail. Top surface 340 of piston 328 is exposed to the workingfluid. Piston 328 has rings 350, 353 for sliding, sealing engagementinside the cylinder as known to those of ordinary skill in the art.Connecting rod 349 has a pair of legs 362, 365 extending downward andslightly outward to second ends 357, 358. Legs 362, 365 have openings363, 366 disposed therethrough for receiving the crankpins 334 or 337.Openings 363, 366 are typically provided with bearing surfaces 367 (FIG.10A) such as bushings or the like as will be known to those of ordinaryskill in the art. Turning to FIG. 10A, connecting rod 349 is connectedto piston 328 by pin 356 mounted inside a ball 359. Connecting rod 349has an opening 354 for receiving the ball 359. The ball 359 has acentral opening for receiving the pin 356. The piston 328 has a centralaxial opening 329 for receiving the first end 351 of the connecting rod349 and has a pair of transverse openings 364, 368 disposed on oppositesides of piston 328 for receiving the pin 356 in the transversedirection (indicated by arrow 369) relative to the piston axis 370. Pin356 is disposed through the opening in the ball 359 and throughtransverse openings 364, 368 in piston 328 and is secured in position bya connecting member 373. As best shown in FIG. 10B, the connectingmember 373 has body 375 with flange 377 at one end 379 and has a cap 381with a flange 383 at one end 385. When the two parts of the connectingmember 373 are attached, the flanges 377, 383 prevent the pin 356 fromsliding out of the ball 359 and the transverse openings 364, 368 in thepiston 328. Body 375 of the connecting member 373 has elongate sections387 extending in the direction of the longitudinal axis 388. Elongatesections 387 have a reduced width section 389 located toward the distalend 390. The reduced width sections 389 extend to fingers 391 that havethe same width as the remainder of the elongate sections 387 for asection 392 and then an angled section 393 terminating at the distal end390. Cap 381 has a cylindrical portion 393 with openings 396 disposedaround the circumference. A ring 397 is formed on the opposite side ofthe openings 396 and the ring 397 terminates at a distal end 399.

Hollow pin 356 is installed through transverse openings 364, 368 in theopposite sides of piston 328 and through ball 359 and is secured byplacing the body 375 of connecting member 373 through the pin 356 andattaching cap 381 to distal end 390 of body 375. When cap 381 is beingengaged with body 375, fingers 391 deflect inward and then snap intoopenings 396 and ring 399 fits into reduced width section 389 on body375.

The ball and pin structure 352 described above provides mechanicaladvantage and reduces the size and weight of connecting rod 349 and pin356. Pin 356 transmits the force received from the pressure chamber intoball 359, and ball 359 transmits the force from pin 356 into crankshaft313. Use of ball 359 instead of a pivot joint allows an additionaldegree of freedom useful in releasing stress from any misalignment.

Other structures for joining the connecting rod 349 to the piston 328may also be suitable as will be evident to those of ordinary skill inthe art.

The first, second, and third embodiments provide several surprisingadvantages. Rotary actuators 100, 200, and 300 have the advantage ofbeing able to be selectively operated in a low backlash mode, whichprovides a higher degree of precision in controlling an output member.Additionally, since the low backlash mode operation is optional,precision operation can be substituted with a low wear mode ofoperation.

Additionally, rotary actuators 100, 200, and 300 have the advantage ofbeing balanced hydraulic actuators. More specifically, in a balancedhydraulic actuator system an equivalent amount of hydraulic fluid entersthe expanding chambers as volume of fluid that is exiting the shrinkingchambers. Having a fluid and force balanced actuator system allows formultiple advantages. Balanced hydraulic systems provide greaterhydraulic pump efficiency. Additionally, hydraulic pumps such as a bentaxis hydraulic pump which are more suited for balanced hydraulicoperation can be used. Further, balanced forces allow for the design ofsimpler servo controllers because the servo control algorithms andhydraulic pressure control valves do not need to account for aright/left force differential.

Rotary actuators 100, 200, and 300 also have the advantage of having avery thin envelope. More specifically, as shown in FIG. 7, thehorizontal width of rotary actuator 300 is much smaller than comparableprior art systems. Since cylinders 328 and 331 are staggered and offsetfrom each other, a thin actuator envelope is achieved that is notpossible if the cylinders 328, 331 are not staggered and offset.Additionally, because each piston connecting rod 349 in rotary actuator300 has a double leg (FIG. 6, 362 & 365) pivot joint connection, and ahigh surface area, ball joint (352), very high forces can be appliedwithout damaging the joints, which in turn allow for a shorter lever armand thin envelope.

Additionally, since the linear actuators only need to be single actingthey provide lower part counts, lower cost, and simpler design incomparison to prior art double acting linear actuators. The singleacting linear motors used in actuators 100, 200, and 300 also providethe advantage of having a low hydraulic leakage rate. More specifically,prior art double action hydraulic pistons typically have a piston linkwhich passes through a high pressure chamber which acts upon one side ofthe piston. Such prior art systems require a high pressure seal acrossthe piston link surface, which are problematic to design and maintain,and often result in significant leakage. Because the only high pressureseals in the disclosed embodiments are between the piston outer surfaceand the cylinder inner surface, there is not a high level of hydraulicfluid leakage as would occur in a prior art piston link seal.

FIGS. 11-15 provide views of a fourth embodiment of the rotary actuator.FIG. 11 is a side cross section view of rotary actuator 400 taken alongsection line 11-11 in the front view FIG. 12. As shown in FIGS. 11-12,rotary actuator 400 includes housing 410, output shaft 413, cylinder419, piston 422, connecting link 448, and slide bearing 447. FIG. 11also shows left end plate 490 and right end plate 491. End plate 490 hasbeen removed in FIG. 12.

Housing 410 is formed of a rigid non-permeable material such as castiron, steel, composite, high strength plastic, or other similarmaterial. Housing 410 provides a surface for bolting or mountingactuator 400 to a reference structure. Cylinder 419 is formed as athrough-bore of housing 410. Cylinder 419 has a generally hollowcylindrical shape with first end 471 and a second end 472. Approximatelyhalfway between first end 471 and second end 472 the upper wall ofcylinder 419 has hole 470.

Piston 422 is arranged and configured for sliding engagement withincylinder 419. As shown in FIG. 11, piston 422 has a generallycylindrical shape with a generally rectangular prism shaped region 401cut into the cylinder. More specifically, as shown in the orientation ofFIG. 11, piston 422 has the general shape of a cylinder arranged on itsside. Piston 422 has a left vertical circular end surface 473 which hasa diameter substantially similar to the inside diameter of cylinder 419.Following a clockwise outer perimeter of cylinder 422, the upper edge ofsurface 473 connects to horizontal cylindrical surface 475. Horizontalcylindrical surface 475 has ridges facing cylinder 419 and is configuredfor holding seals between piston 422 and cylinder 419. Such seals arering shaped and made from Teflon or some other similar material.Cylindrical surface 475 extends rightwards to connect to annularvertical surface 476. Annular vertical surface 476 extends downwards toflat horizontal surface 477. Flat horizontal surface 477 extendsrightwards to connect to semi cylindrical surface 478 which has acylindrical axis oriented perpendicular to the page as shown in FIG. 11.Surface 478 extends first downwards, then rightwards, and back upwardsto flat horizontal surface 479. Surface 479 is parallel and in the sameplane as surface 477. Surface 479 extends rightwards into annularvertical surface 480. Annular surface 480 has an outer diameter. Thisouter diameter is substantially equal to the diameter of cylinder 419.Surface 480 extends upwards to connect to horizontal cylindrical surface481. Horizontal cylindrical surface 481 also has ridges facing cylinder419 and configured for holding seals. Surface 481 extends rightwards tovertical circular surface 474. Surface 474 extends downward and connectsback to cylindrical surface 481 at 482. As shown in FIGS. 11, 481 and482 are pointing to the same cylindrical surface cut by the sectionplane. The surface pointed at by 481 and 482 are also the same surfaceas pointed to at 475 and 483. The surface at 482 extends leftwards to483. The surface at 483 makes contact with the lower end of verticalcircular surface 473, completing a clockwise perimeter walk aroundpiston 422.

Passing through the central region of cylindrical surface 478 isvertical through-bore 485. Arranged in close tolerance withincylindrical surface 478 is cylindrical slide bearing 447. Slide bearing447 is free to slide against piston surface 478 in two degrees offreedom including lateral sliding into and out of the page (as orientedin FIG. 11) and also rotation about the axis of cylindrical surface 478.

Slide bearing 447 is a generally cylindrical shape with its cylindricalaxis coaxial with the cylindrical axis of surface 478. Slide bearing 447has cylindrical through-bore 486, which holds lower end 448 a of rodshaped connection link 448 in sliding engagement. More specifically,link 448 is able to slide relative to slide bearing 447 along line 487.Connecting link 448 extends through hole 470 where it connects withoutput member 413, and continues extending its upper end 448 b intochamber 403. Chamber 403 is defined by an upper wall of housing 410, endplates 490 and 491, and upper wall of cylinder 419. Chamber 403 is influid communication with hole 470, region 401, and fluid port 499 whichis arranged in the upper wall of housing 410.

Output member 413 is arranged spanning hole 470 and is coupled to pivotjoint 414. Pivot joint 414 allows output member 413 to rotate relativeto housing 410 about an axis 415 directed perpendicular to the page asshown in FIG. 11. Output member 413 has cylindrical through-bore 488which forms a sleeve around link member 448 holding link 448 in tightnon-moving engagement. Pivot joint 414 is also coupled to link 448,causing movement of link 448 to be limited to rotary movement about axis415.

Arranged on left and right ends of cylinder 419, and attached to housing410, are end plates 490 and 491 respectively. Hydraulic port 492 passesthrough end plate 490 to connect to chamber 494 which is formed bycylinder 419 and piston surface 473. Similarly, hydraulic port 493passes through end plate 491 to connect to chamber 495 which is formedby cylinder 419 and piston surface 474. Around upper end 448 b of link448 is slide bearing 447′. Slide bearing 447′, which is substantiallysimilar to slide bearing 447, is only shown for demonstrative purposesin FIGS. 11-15. This embodiment does not have slide bearing 447′, butFIGS. 11-15 show how easily slide bearing 447′ can be added togetherwith a second piston symmetrical to piston 422 in chamber 403.

FIGS. 13-15 show views of the piston assembly shown in FIG. 11,including piston 422, connecting link 448, pivot joint 414, and slidebearing 447. Note that in FIGS. 13-15 connecting link 448 is in avertical orientation, whereas in FIGS. 11 and 12, connecting link 448 isin a rotated configuration.

Comparing the changes from FIG. 11 to FIG. 15, it can be observed howslide bearing 447 has rotated counter clockwise, and that connectinglink lower end 448 a has slid downwards relative to slide bearing 447,penetrating into bore hole 486.

Rotary actuator 400 generally operates by adjusting the hydraulicpressures in ports 492 and 493 to cause piston 422 to move leftwards orrightwards, which in turn causes connecting link to act as a rotatinglever, which then causes output link 413 to also rotate.

As an example, we consider rotary actuator 400 being in a state as shownin FIG. 11 in which it is desired to rotate output member 413 counterclockwise. First, ports 492 and 493 would be connected to hydrauliccontrol lines, housing 410 would be mounted on a reference structure,and output shaft/link 413 would be connected to a member to berotationally driven. The hydraulic pressure in port 492 would then beincreased while the hydraulic pressure in port 493 is decreased. Thiscauses the pressure in chamber 494 to increase, and the pressure inchamber 495 to decrease. When the pressure in chamber 494 falls belowthe pressure in chamber 495, a net rightwards force is effectivelyapplied on piston 422. More concretely, the pressure placed by the fluidin chamber 494 applies a rightwards force on circular surface 473. Asimilar leftwards force is created by the pressure in chamber 495 oncircular surface 474. Since the pressure in 494 is greater than 495, therightwards force is greater than the leftwards force, resulting in a netrightwards force applied to piston 422. This force is effectivelymediated on piston 422 through housing 410 and endplates 490 and 491.

The rightwards force on piston 422 is communicated as a rightwards forceon lower end of connecting link 448 a through slide bearing 447. Becauseconnecting link 448 is rigidly coupled to output shaft 413, and becauselink 448 and output shaft 413 are coupled to pivot joint 414, connectingrod 448 can only move as a rotation about pivot joint 414. Therightwards force applied to connecting link 448 causes connecting link448 to act as a lever with a fulcrum at pivot joint 414. Therefore, therightwards force applied by piston 422 is converted into acounterclockwise torque on connecting link 448 which is then passed tooutput shaft 413.

As piston 422 slides rightwards relative to housing 410, connecting link448 rotates counterclockwise relative to housing 410. As connecting link448 rotates counterclockwise, the bottom end 448 a of link 448 mustslide downwards relative to sliding bearing 447. In other words, sincelink bottom end 448 a must travel in an arc relative to pivot joint 414,the vertical height of bottom end 448 a relative to piston 422 mustchange as the angle of rotation of connecting link 448 changes. Also, asconnecting link 448 rotates counterclockwise relative to housing 410,slide bearing 447 must also rotate counterclockwise relative to piston422 since connecting link 448 is encircled in low tolerance by slidebearing 447.

If there is any error in the alignment between the connecting link 448,piston 422, and cylinder 419, slide bearing 447 is free to slide into orout of the page as shown in FIG. 11 in order to relieve suchmisalignment. For example, if cylinder 419 is not perfectly orthogonalto the plane that connecting link 448 rotates in, such as if the rightend of cylinder 419 tilts slightly up out of the page, slide bearing 447will be able to slide upwards/downwards as piston 422 moves left andright in order to maintain unstrained contact with connecting link 448.

Because the cross section of cylinder 419 is the same on the left sideof piston 422 as on the right side of piston 422, the volume of fluidwhich must flow in through port 492 must be equal to the volume of fluidflowing out of port 493 for a rightwards movement of piston 422. Thus,rotary actuator 400 is a balanced hydraulic actuator.

The seals arranged between piston 422 and cylinder 419 at 475 and 481,prevent high pressure from chambers 494 and 495 from passing intoregions 401 and 403. Thus, the output shaft 413 does not come intocontact with any high pressure chamber. Port 499 is used to supply oilwhich may be needed to lubricate output shaft 413 and connecting link448, or to drain any oil which leaks across the seals between piston 422and cylinder 419.

FIGS. 18-21 show a variation of rotary actuator 400 having a secondversion piston assembly 505 in which cylindrical slide bearing 447 isreplaced with ball slide bearing 547. FIG. 18 is a perspective view ofsecond version piston assembly 505 showing piston 522, holding ballslide bearing 547, which embraces connecting link 548. FIG. 19 is a topview of assembly 505 showing the arrangement of the ball slide bearing547 in piston 522. FIG. 20 is a side view of piston assembly 505, andFIG. 21 is a sectional side view taken along line 21-21 in FIG. 19. Asshown in FIG. 21, ball slide bearing 547 is held in two dimensionalrotary engagement to piston 522 through race 507. Race 507 is held inpiston 522 by a flanged annular end stop 506. Ball slide bearing 547allows the lower end 548 a of connecting link 548 to linearly slide inand out of the central through-bore of the ball slide bearing 547.

Ball slide bearing 547 operates very similarly to cylindrical slidebearing 447 in rotary actuator 400. However, as viewed in FIG. 21,instead of providing a degree of freedom into and out of the pagerelative to the piston like slide bearing 447, ball slide bearing 547provides two degree of freedom, rotary movement between ball slidebearing 547 and piston 522. This second degree of freedom relieves anymisalignment of connecting link 548 in which connecting link 548 is notperfectly arranged in the plane of the page shown in FIG. 21.

FIGS. 16-17 show another embodiment of the rotary actuator that issimilar to rotary actuator 400 but having four cylinders and fourpistons. FIG. 16 is a top view of actuator 600 with end plates removedshowing parallel cylinders 619 a, 619 b, 619 c, and 619 d. FIG. 17 is aside cross section view taken along line 17-17 in FIG. 16. As shown inFIG. 17, pistons 622 a and 622 b are arranged in respective cylinders619 a and 619 b. Piston 622 a and piston 622 b are substantiallysymmetrical about the axis of pivot joint 614. When a clockwise torqueis desired on output shaft 613, piston 622 a is driven rightwards whilepiston 622 b is driven leftwards. The hydraulic ports driving pistons622 a and 622 b may or may not be hydraulically coupled. If they arehydraulically coupled, hydraulic phasing will be easier. If they are nothydraulically coupled, while phasing may be more difficult, the systemwill be redundant if one of the piston-cylinder pairs is hydraulicallycompromised. While not shown, pistons 622 c and 622 d are planarsymmetric to pistons 622 a and 622 b. All four pistons are coupled tooutput shaft 613.

The embodiments 400 and 600 have several surprising advantages overprior art rotary actuator systems. Rotary actuators 400 and 600, likeactuators 100, 200, and 300 have the advantage of being balancedhydraulic actuators. For example, with reference to FIG. 11, as piston422 moves rightwards, the volume of fluid entering chamber 494 issubstantially equal to the volume of fluid exiting chamber 495. In aprior art double acting piston which has a piston rod passing throughone chamber, the volume of fluid entering/exiting the piston rod sidechamber would be less than the fluid exiting/entering the non piston rodchamber due to the cross sectional area of the piston rod. Additionally,the cross sectional area of the piston rod would cause the force that isapplied to a piston for a given hydraulic pressure to be different onthe side of the piston without the piston rod. Because rotary actuator400 has no piston rod which passes through chambers 494 or 495, themagnitude of force applied to piston 422 for a given pressure in chamber494 is equivalent to the opposite force which would be applied bychamber 495 placed at an equivalent pressure. Having a fluid and forcebalanced actuator system allows for multiple advantages. Balancedhydraulic systems provide greater hydraulic pump efficiency.Additionally, hydraulic pumps such as a bent axis hydraulic pump whichare more suited for balanced hydraulic operation can be used. Further,balanced forces allow for design of simpler servo controllers becausethe servo control algorithms and hydraulic pressure control valves donot need to account for a right/left force differential.

Additionally, rotary actuators 400 and 600 have the advantage of havinga thin profile and low part count as found in actuators 100, 200, and300. A thin profile allows these actuators to be used in thin wingaircraft designs or other environments requiring a thin profile.

As shown in FIG. 22, multiple rotary actuators 300 can be combined todrive the same output member 316 in order to achieve a high drivetorque, or a fault tolerant/redundant system. The actuators 300 areshown with their housing removed for clarity. The actuator shafts 313 ofthe two actuators 300 are connected such that the shafts 313 of the twoactuators 300 form a single unit that can be acted on by all fourpistons simultaneously.

FIG. 23 shows an actuator system 700, which includes one or more rotaryactuators 720 and an electro hydraulic bent axis pump system 730. System700 also includes hydraulic reservoir 725 and servo valve system 722.Pump 730 is specifically designed for efficient operation in a balancedhydraulic system.

FIG. 24 provides a side section view of electro hydraulic bent axis pumpsystem 730. Pump system 730 includes the major components of bent axispump 740, gear box 750, electric motor 760, and housing 731, which holdseach of the other components together. Pump system 730, when driven,creates a pressure differential and fluid flow between hydraulic port733 and 735.

Bent axis pump 740 contains piston heads 737 a and 737 b which areconnected to piston links 738 a and 738 b respectively. Piston heads 737a and 737 b are arranged within a pump body supported by bearings 739.Piston links 738 a and 738 b are coupled to rotor 744, which issuspended by bearings 741.

Gear box 750 contains gears 751, 752, and 753. Gears 751, 752, and 753are held in housing 731. Gearbox 750 is mechanically coupled to rotor744. Motor 760 has output shaft 761 which is coupled to gear box 750.Motor 760 also has stator 762 and rotor 763. Gear box 750 is configuredto provided a mechanical advantage which causes bent axis pump rotor torotate at a speed lower than motor shaft 761.

Pump system 730 is particularly suited to use with balanced hydraulicactuators. Because bent axis pump 740 only has two ports, it isparticularly suited to balanced hydraulic actuators which would notcause a need for a third hydraulic port for increasing or decreasing thehydraulic fluid volume of the system. Further, the use of the gear boxproviding a mechanical advantage allows for prolonged pump systemlifetimes, which is particularly appropriate for aircraft applications.

The particular embodiments shown may also be combined with a servocontroller. A standard servo controller can be used to control thelinear motors to adjust their force or position output based upon acommanded output member torque/position and a measured output membertorque/position.

Therefore, while the presently-preferred form of the rotary actuator,rotary actuator system, and method of operating a rotary actuator aredisclosed and described, and several modifications discussed, personsskilled in this art will readily appreciate that various additionalchanges may be made without departing from the scope of the invention.

1. A rotary actuator comprising: a reference structure; an output memberarranged for rotary movement relative to said reference structure aboutan axis; a first linear motor having a first member and a second member,said first linear motor configured and arranged to selectively apply anoutput force urging said first member and said second member apart alonga generally linear direction; said first linear motor first membercoupled to said reference structure; said first linear motor secondmember coupled to said output member and configured and arranged tocause a torque between said output member and said reference structurein a first direction about said axis when said first linear motorapplies said output force; a second linear motor having a first memberand a second member, said second linear motor configured and arranged toselectively apply an output force urging said second linear motor firstmember and said second linear motor second member apart along agenerally linear direction; said second linear motor first membercoupled to said reference structure; and said second linear motor secondmember coupled to said output member and configured and arranged tocause a torque between said output member and said reference structurein a direction about said axis opposite said first direction when saidsecond linear motor output force is applied.
 2. A rotary actuator as setforth in claim 1, wherein said first linear motor comprises a singleacting hydraulic motor.
 3. A rotary actuator as set forth in claim 2,wherein said first linear motor first member comprises a prismaticchamber and said first linear motor second member comprises a piston. 4.A rotary actuator as set forth in claim 3, wherein said prismaticchamber is a cylinder.
 5. A rotary actuator as set forth in claim 3,wherein said first linear motor comprises a piston link arranged betweensaid piston and said output member.
 6. A rotary actuator as set forth inclaim 3, wherein said first linear motor first member is rigidly mountedto said reference structure.
 7. A rotary actuator as set forth in claim5, wherein said piston link and said piston are connected through a balljoint.
 8. A rotary actuator as set forth in claim 5, wherein said pistonlink and said output member are connected through a pivot joint or pinjoint.
 9. A rotary actuator as set forth in claim 1, wherein said firstlinear motor and said second linear motor each have a direction ofaction which is generally parallel.
 10. A rotary actuator as set forthin claim 1, wherein said output member comprises a shaft and a firstpivot bearing coupled to said first motor second member and a secondpivot bearing coupled to said second motor second member.
 11. A rotaryactuator as set forth in claim 10, wherein said first pivot bearing andsaid second pivot bearing are separated by an offset in a dimensionparallel to said axis.
 12. A rotary actuator as set forth in claim 10,wherein said first pivot bearing, said second pivot bearing, and saidaxis are collinear.
 13. A rotary actuator as set forth in claim 1,wherein said first linear motor first member comprises a cylinder andsaid first linear motor second member comprises a piston, said pistoncomprising a first surface and a second surface, said first surfaceforming a first chamber with said cylinder and said second surfaceforming a second chamber with said cylinder.
 14. A rotary actuator asset forth in claim 13, wherein said cylinder comprises a generallycylindrical surface, and said cylindrical surface comprises a holebetween said piston first surface and said piston second surface.
 15. Arotary actuator as set forth in claim 14, and further comprising a drivelink coupled to said piston and traversing said hole.
 16. A rotaryactuator as set forth in claim 2, wherein said second linear motorcomprises a single acting hydraulic motor.
 17. A rotary actuator as setforth in claim 16, wherein said first linear motor and said secondlinear motor have an equivalent hydraulic fluid volume displaced for agiven linear motor linear distance of actuation.
 18. A rotary actuatoras set forth in claim 16, wherein said first linear motor and saidsecond linear motor are hydraulically balanced.
 19. A rotary actuator asset forth in claim 1, wherein said output member is coupled to anaircraft control surface.
 20. A rotary actuator as set forth in claim 1,and further comprising a position sensor configured and arranged tomeasure an angle between said output member and said referencestructure.
 21. A rotary actuator as set forth in claim 20, and furthercomprising a servo controller.
 22. An actuator for rotating a shaftabout an axis, said actuator comprising: a housing; a first singleacting cylinder disposed in said housing and having a first piston and afirst connecting link therein; a crank disposed on said shaft; a secondsingle acting cylinder disposed in said housing and having a secondpiston and a second connecting link therein; wherein said first andsecond connecting links are attached to different locations on saidcrank; and wherein said actuator is configured and arranged such thatactuation of said first single acting cylinder causes said crank torotate in a first direction and actuation of said second single actingcylinder causes said crank to rotate in a second direction opposite saidfirst direction.
 23. The actuator of claim 22, wherein said first andsecond cylinders are oriented substantially parallel.
 24. The actuatorof claim 22, wherein said first and second cylinders are both configuredand arranged to each have a pre-load to provide a force in the samegeneral direction to remove a backlash.
 25. The actuator of claim 22,wherein said shaft rotates on a set of bearings disposed in saidhousing.
 26. The actuator of claim 22, wherein said shaft is connectedto an aircraft control surface.
 27. The actuator of claim 22, whereinsaid actuator is configured and arranged to move said crank from a firstposition to a second position by applying an additional pressure to oneof a first pressure chamber and a second pressure chamber.
 28. Theactuator of claim 27, wherein said actuator is configured and arrangedto be able to maintain a position of said crank by providing asubstantially equal pressure inside said first and second pressurechambers.
 29. The actuator of claim 27, wherein said actuator isconfigured and arranged to maintain a position of said crank by notallowing hydraulic fluid to flow in or out of said first or secondpressure chambers.
 30. The actuator of claim 22, wherein said first andsecond single acting cylinders have a cross section which is notcircular.
 31. The actuator of claim 22, wherein said first connectinglink is connected to said first piston through a ball joint.
 32. Theactuator of claim 22, wherein said first connecting link and said outputmember are connected through a pivot joint or pin joint.
 33. Theactuator of claim 22, wherein said first and second single actingcylinders are separated by an offset in a dimension parallel to saidaxis.
 34. The actuator of claim 22, wherein said first and second singleacting cylinders share a common bore.
 35. The actuator of claim 22,wherein said actuator is configured and arranged such that said firstsingle acting cylinder expels a substantially equal volume of hydraulicfluid to a volume of hydraulic fluid drawn in by said second singleacting cylinder for a movement of said shaft.
 36. The actuator of claim22, and further comprising a position sensor configured and arranged tomeasure an angle between said shaft and said housing.
 37. The actuatorof claim 36, and further comprising a servo controller. 38-56.(canceled)